A power converter, such as a Direct Current (DC) to DC power converter for a microprocessor, may need to provide precise output voltage control (e.g., any output voltage ripple may need to be small), high efficiency, and fast transient response (e.g., the speed at which the converter responds when an output voltage rises too high or falls too low). Similarly, a maximum input current ripple and an overall size requirement may be imposed on the converter. Moreover, operating supply voltages for microprocessors and associated circuits have decreased, resulting in increased supply currents. The costs required to support these increased currents (e.g., via motherboards, sockets, and/or packages) have also increased.
One approach to providing an appropriate power converter is to use several smaller converter modules, each operating at the same frequency but at different phases. For example, FIG. 1 is a diagram of a traditional Pulse Width Modulated (PWM) multi-phase DC-DC converter 100 that senses output voltage (VOUT) and output current. In particular, a voltage controller 110 compares VOUT with a reference voltage (VREF) to generate a voltage error signal. The voltage error signal is then used as a current reference by each converter module 120 (e.g., each of the four converter modules 120 illustrated in FIG. 1).
A current controller 130 compares a module""s output current with the reference current and generates a control signal that is used by a PWM modulator to determine a duty cycle to drive a bridge (e.g., via a driver 140). Each PWM modulator includes a comparator 150 and a phase shifter 160 that receives a sawtooth waveform from an oscillator 170 (e.g., a waveform having a relatively large amplitudexe2x80x94such as several volts). In particular, the four phase shifters 160 illustrated in FIG. 1 shift the waveform by 0, 90, 180 and 270 degrees. Note that the waveform from the oscillator 170 is injected after VOUT and VREF have been amplified.
Such a PWM converter 100 may provide small input current ripple and output voltage ripple. This approach, however, can suffer from slow transient response (e.g., orders of magnitude slower than what might be required for a microprocessor).
As another approach, a hysteretic converter (e.g., a xe2x80x9cbang-bangxe2x80x9d converter) may provide faster transient response. For example, FIG. 2 is a diagram of a traditional hysteretic DC-DC converter 200. Note that the converter""s feedback loop includes a generic switching bridge 300. FIG. 3 illustrates some known switching bridges 310, 320. In particular, the first switching bridge 310 comprises a p-channel Metal-Oxide-Semiconductor (PMOS) device and a diode. The second switching bridge 320 comprises a PMOS device and an n-channel MOS (NMOS) device.
Referring again to FIG. 2, the hysteretic DC-DC converter 200 further includes a hysteretic comparator 210 that receives a reference voltage (VREF) via a first input. If an output voltage (VOUT) crosses one of the thresholds determined by the hysteresis window around VREF, the comparator 210 flips and rapidly pulls VOUT inside the window. For example, if VOUT drops below the window""s lower threshold, the control circuit turns on the switching bridge 300 and connects the inductor 220 to the positive input power supply. Despite the advantage of a fast transient response, however, the switching frequency of the converter 200 is sensitive to circuit component parameters (e.g., there is no provision for externally setting the phase and frequency at which the converter 200 operates). As a result, the switching times of several identical converters 200 operating in parallel may be substantially the samexe2x80x94causing them to operate as a large single-phase converter (and producing a large output voltage ripple).
It is also known that voltage mode control may be used to reduce the sensitivity of a traditional hysteretic DC-DC converter 200 (e.g., by using an error amplifier to compare VOUT and VREF). This approach, however, may reduce the transient response time of the circuit. Similarly, it is know that V2 mode control may be used to improve load current transient characteristics. In this case, however, the switching frequency and stability of the circuit may depend on output filter characteristics, such as the Equivalent Series Resistance (ESR) and the Equivalent Series Inductance (ESL) of an output capacitor, and any stray inductance and/or resistance associated with the supply path.